The invention relates to encapsulated electrical circuits for ink jet printheads and to improved encapsulating methods for protecting circuits for ink jet printheads.
Ink jet printers continue to evolve as the technology for ink jet printing continues to improve to provide higher speed and higher quality printers. The improvement in speed and quality does not come without a price, however. The printheads of such printers are more costly to manufacture and thus there is a movement to longer life permanent or semi-permanent printheads that are used in conjunction with replaceable ink cartridges.
With the movement to longer life permanent or semi-permanent printheads, there arises a need for improved methods of protecting the electrical components, particularly the flexible circuit or tape automated bonding (TAB) circuit traces or TAB beams and the electrical connections between the TAB beams and the substrate chips from ink corrosion. During the life of the printheads, components of the printhead are subjected to mechanical, thermal, electrical, and chemical stresses. In particular, the TAB circuit or flexible circuit connections to the printhead chip may fail due to ink corrosion of the connections. Therefore it is important to adequately protect the electrical connections from the ink, otherwise electrical shorts or inadequate electrical signals to the printhead could result.
One method for protecting ink jet printheads includes causing a die bond adhesive used to attach the printhead chip to the printhead body to flow up along the sides of the chip to contact the back side of an electrical circuit attached to the chip. An epoxy encapsulant is then applied to the top side of the chip after curing the die bond adhesive. Because of slight production variations, the method is inadequate to assure that sufficient die bond adhesive to protect the electrical circuit has flowed up along side of the chip for each printhead.
Another method includes applying a first epoxy layer to the back side of the circuit assembly after attaching the electrical circuit to the printhead chip. This epoxy layer is cured and the chip is attached to a printhead body using a die bond adhesive. A second epoxy material is applied to the top side of the circuit assembly and cured after curing the die bond adhesive. In this assembly technique, the electrical leads are sandwiched between two cured epoxy layers creating an interface at the crucial area joining the two layers. This epoxy interface is prone to delamination thereby permitting ink to flow between the layers and contact the electrical connections.
Despite advances made in the art of bonding electrical circuits to pen bodies, there remains a need for improved methods that provide enhanced protection of electrical components and greater flexibility with regard to manufacturing processes.
With regard to the foregoing and other object and advantages, the invention provides a method for encapsulating electrical traces on a flexible circuit or TAB circuit and electrical connections between the flexible circuit or TAB circuit and a printhead substrate to inhibit ink corrosion thereof. The method includes applying a first adhesive to a first surface of the flexible circuit or TAB circuit. A second adhesive is applied to first surface of the flexible circuit or TAB circuit whereby the first and second adhesives effectively coat exposed portions of the traces and connections on the first surface and on a second surface of the flexible circuit or TAB circuit. The first and second adhesives are preferably thermally curable epoxy compositions that are miscible with each other and the first adhesive has a first viscosity and first thixotropy index that is lower than a second viscosity and second thixotropy index of the second adhesive. After applying the adhesives, the first and second adhesives are cured.
In another aspect the invention provides a printhead for an ink jet printer. The printhead includes a cartridge body containing at least one printhead substrate adhesively attached to the cartridge body and flexible circuit or TAB circuit electrically connected to the printhead substrate. The flexible circuit or TAB circuit contains electrical traces and electrical connections from the traces to bond pads on the printhead substrate. A thermally cured encapsulant encapsulates the connections and traces. The encapsulant is derived from first and second adhesives. The first adhesive is a thermally curable epoxy composition having a first viscosity and first thixotropy index sufficient for flow of the first adhesive between the connections and traces from a first surface of the flexible circuit or TAB circuit to a second surface of the flexible circuit or TAB circuit. The second adhesive is a thermally curable epoxy composition miscible with the first adhesive and having a second viscosity and second thixotropy index whereby flow of the second adhesive through the connections and traces is substantially inhibited.
An advantage of the compositions and methods according to the invention is that ink jet pens may be assembled and critical electrical connections protected with an encapsulant that is less prone to delamination under mechanical, chemical, thermal or electrical stresses since both adhesive materials used as an encapsulant are cured at substantially the same time. This is particularly important for protecting critical electrical components in permanent or semi-permanent printheads. Another advantage of the invention is that by using adhesives having different viscosities and thixotropy indices, both surfaces of the flexible circuit or TAB circuit can effectively be protected from ink corrosion by applying the adhesives to only one surface of the flexible circuit or TAB circuit. The adhesives are also adaptable to application to the printhead after the printhead substrate has been fixedly attached to the cartridge body thus eliminating the need to apply an adhesive or encapsulant to the back side of a flexible circuit or TAB circuit prior to attaching the printhead substrate and flexible circuit to a cartridge body.
Thixotropic adhesives are adhesives which exhibit a time-dependent viscosity. In other words, the adhesives are substantially liquid materials which gell on standing, then thin out when stirred and again gell when the stirring stops. For the purposes of this invention, the xe2x80x9cthixotropy indexxe2x80x9d of an adhesive is a ratio of the viscosities of the adhesive at two different shear rates determined using a BROOKFIELD model HBT viscometer.